Understanding the unintended spread and impact of alien and invasive fish species – development of management guidelines for South African inland waters Report to the WATER RESEARCH COMMISSION by PK Kimberg, DJ Woodford, OLF Weyl, C Hui, DM Richardson, TP Msezane, KA van der Walt, ER Swartz, CT Chimimba, T Zengeya, BR Ellender WRC Report No. 2039/1/14 ISBN 978-1-4312-0587-5 September 2014 Obtainable from Water Research Commission Private Bag X03 Gezina, 0031 [email protected] or download from www.wrc.org.za DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. © Water Research Commission ii EXECUTIVE SUMMARY BACKGROUND Invasive alien species are recognised globally as direct drivers of biodiversity and ecosystem service loss that pose substantial threats to fragile ecosystems and threatened and endangered species. In South Africa, this recognition led to the promulgation of the National Environmental Management: Biodiversity Act (Act 10 of 2004) and the subsequent supporting Alien Invasive Species Regulations (RSA 2013 and amended in 2014). These regulations provide a legal framework for the future management of invasive alien species, including freshwater fishes, within the borders of South Africa. The Alien Invasive Species regulations now recognise 3 distinct categories of invasive fish species each with prescribed management obligations. RATIONALE In South Africa some invasive alien fish species such as bass, carp and trout threaten biodiversity and sensitive ecosystems in some areas but are important contributors to the national economy elsewhere. It is also recognised that in many cases alien species are so well established that their eradication would not be feasible. As a result, some alien fishes should be actively managed to limit further impacts on biodiversity, as well as to maximise social and economic benefits from the resource in areas where they are already established. This will require a national management plan that will guide eradication in areas of biodiversity concern (e.g. protected areas) and permit their use in others. The rationale for this project was to use case studies that exemplify the issues surrounding the understanding of alien fish invasion processes and impacts in order to collate key lessons learned that would inform a management decision framework. The key processes for the various case studies were: • Sundays River: Obtaining increased understanding of processes of transport and establishment of invasive alien species especially through water transfer infrastructure; • Groot Marico catchment: Understanding interspecific impacts and conservation implications for native species; and • Other case studies: Determining invasion state, assessing impacts over time in unstable systems, determining best practices for eradication operations. OBJECTIVES AND AIMS The overarching objectives of this study were: iii • To conduct two comprehensive case studies that exemplify the South African situation regarding the spread and impacts of alien invasive fishes in aquatic ecosystems; and • To use the results of these case studies to develop a decision support framework to prioritise areas for alien fish eradication efforts within river systems and better manage the impacts of human mediated transfer of alien fish. CASE STUDY 1: ASSESSING IMPACTS OF LARGEMOUTH BASS IN A NEAR-PRISTINE ECOSYSTEM The catchment of the Groot Marico River is characterised by deeply incised gorges that are fairly inaccessible and unsuitable for large-scale agriculture or development, and therefore uniquely un-impacted compared to most other rivers in the North West Province or in South African. The Groot Marico is also relatively unique in the South African context in that it originates in a series of dolomitic eyes. The dolomitic eyes discharge into tributaries that confluence to form the Groot Marico main stem. Studies conducted during the 1980s and 1990s identified the significant conservation value of the dolomitic eye ecosystems and their associated biota. Based on those assessments the tributaries of the Groot Marico River were identified as biodiversity features of special ecological significance. As a result, these streams are prioritised in the provincial conservation plan and the entire upper catchment is classified as a Freshwater Priority Area (FEPA) and Fish Support Area. This study aimed to assess the impact of Largemouth bass (Micropterus salmoides), an invasive alien fish species, on the relatively unimpacted fish community in the Groot Marico catchment. Based on this assessment the endemic Marico barb (Barbus motebensis) was the only species that was strongly negatively correlated with the presence of M. salmoides. The density of B. motebensis showed significant differences between invaded and un-invaded pools within the elevations at which M. salmoides occurred. The remainder of the fish species either showed no correlation or were positively correlated with the presence of M. salmoides. The absence of M. salmoides from the upper reaches of the Groot Marico catchment was attributed to the increased elevation, increasing gradient and barriers to upstream migration. With the exception of the Sterkstroom/ Polkadraaispruit and a seemingly isolated population in the Marico eye (Kaaloog se Loop) M. salmoides was absent from the tributaries. The assessment of trophic interactions between alien and indigenous fish species did not show any discernable difference between invaded and un-invaded sites that could be attributed to the presence of M. salmoides. The only observed evidence of the potential impact of the invasive M. salmoides on indigenous fish communities was the absence of B. motebensis from river section where M. salmoides occurred. iv The results of the genetic assessment identified two distinct lineages (the Draai and Eastern lineages) of B. motebensis suggesting historical isolation between these populations. There were also significant levels of differentiation between tributary populations of Amphilius uranoscopus resulting in the identification of two lineages (the Ribbok and Western lineages). Priority areas for conservation were identified based on the current conservation status of the species (B. motebensis being Vulnerable (VUL) on the IUCN Red List), the number of Evolutionary Significant Units for each species as well as the overall genetic diversity of the species. In total, four tributary populations were identified as conservation priorities areas, these were the Draaifontein, Van Straatens, Ribbokfontein and Kaaloog se Loop tributaries. CASE STUDY 2: UNDERSTANDING THE ROLE OF WATER-TRANSFER INFRASTRUCTURE IN ENABLING INVASION This case study assessed the processes of invasion using a natural experiment provided by the irrigation water transfer infrastructure of the Sundays River valley, a citrus-growing area with a complex canal network that supplies water from an Inter-Basin Water Transfer (IBWT) scheme to approximately 400 small off-channel impoundments. A year-long study linked the number of propagules (immigrating fish) being supplied by the irrigation network to 30 water storage ponds of varying ages, to assess the patterns and processes of fish invasion through water transfer infrastructure. The project showed that high propagule size in the canals corresponds to rapid colonisation of the ponds, provided the introduced fish species is capable of reproducing within the environment of the pond. Although we set out to assess what management strategies could mediate the transport of fish through the canals, we determined that this was impossible, as no change in management strategy or modification of water-transfer infrastructure was likely to result in decreased transfer of fish propagules or meaningful improvements to the conservation status of the freshwater ecosystems. We did, however, identify an on-going invasion of the adjacent Addo Elephant National Park, which was likely enabled through water discharges from the irrigation network. While prevention of this invasion is no longer possible, we describe how an assessment of the risk of invasion and subsequent investment into infrastructure to prevent it (i.e. a weir placed on the tributary upstream of the canal discharge point) may have historically prevented the invasion. We conclude the case study by emphasising the importance of considering the implications of organism transfers when designing inter-basin or intra-basin water transfer schemes, and in investing in preventative measures prior to water transfer infrastructure becoming operational. v OTHER CASE STUDIES We use three case studies to explore other aspects of fish invasions, impacts and management not explicitly covered by the two main case studies summarised above. We describe the use of a unified framework to define invasion state in assessing the status of an invasion of a loricariid catfish in the Richards Bay area of KwaZulu-Natal. We emphasise the benefits of using the framework to establish the extent of the invasion and to prioritise management options. We assessed multiple species invasions in the Swartkops River system in the Eastern Cape, which consisted of natural expansions up a seasonally flowing tributary (the Blindekloof stream) by alien fish species from a common invasion source (the Swartkops